1. Field of the Invention
The present invention relates to a developing apparatus using a two-component developer constituted by a toner and a carrier, and an image forming apparatus such as a copier, facsimile, or printer, which uses the developing apparatus.
2. Description of the Background Art
A conventional developing apparatus uses a developing method in which a developer is conveyed by a developer carrier, a voltage is applied between an image carrier carrying an electrostatic latent image and the developer carrier, and toner is moved onto the electrostatic latent image to reveal the electrostatic latent image. An example of this developing method is a magnetic brush developing method using a two-component developer. In this developing method, a two-component developer constituted by a non-magnetic toner and a magnetic carrier is used, and a magnetic brush is formed by the magnetic carrier on the surface of a developing sleeve serving as a developer carrier having magnets disposed in its interior. The magnetic brush is then rubbed against or brought close to a photosensitive body, i.e. a latent image carrier opposing the developing sleeve with a narrow developing gap (doctor gap) therebetween. Here, development is performed by applying a development voltage between the developing sleeve and the photosensitive body such that toner particles carried on the magnetic carrier forming the magnetic brush on the developing sleeve are adhered to the latent image on the photosensitive body.
In this development operation, the toner is gradually consumed, and therefore the developing apparatus must be replenished with new toner. The replenishment toner is mixed with the magnetic carrier while being conveyed together with the magnetic carrier, and through this mixing operation, a charge is applied to the replenishment toner. If mixing is not performed sufficiently up to the point at which the replenishment toner is supplied to the developing sleeve, charge-deficient toner that has not been charged sufficiently is used for development. When charge-deficient toner is used for development, the charge-deficient toner contained in the developer carried on the developing sleeve scatters, leading to toner scattering. The reason for this is that the charge-deficient toner that has not been sufficiently charged has poor adsorbability relative to the magnetic carrier, and therefore separates from the magnetic carrier and scatters easily. Charge-deficient toner in the developing apparatus that is not carried on the developing sleeve also separates from the magnetic carrier easily. As a result, the charge-deficient toner may fly up inside the developing apparatus and leak out through gaps in the developing apparatus on the periphery of the developing sleeve. In this case also, toner scattering occurs.
When toner scattering occurs, the interior of the apparatus is contaminated as toner adheres to other members on the periphery of the developing apparatus. Toner scattering also occurs when toner other than the replenishment toner is charge-deficient, and this toner scattering also contaminates the interior of the apparatus.
For example, Japanese Patent Publication No. 3,391,926 (Prior Art 1) describes a developing apparatus comprising a developing unit having a developer carrier and a developer supply unit for supplying a two-component developer to the developer carrier, and a stirring unit for stirring the two-component developer and circulating the two-component developer to and from the developing unit. The two-component developer is stirred and conveyed to the developing unit by the stirring unit, and therefore the opportunities for stirring increase such that the occurrence of charge-deficient toner and toner scattering can be suppressed. Note that in the developing apparatus of Prior Art 1, toner replenishment corresponding to toner consumption during the development operation is performed in the stirring unit.
However, in the developing apparatus of Prior Art 1, the two-component developer circulates between the developing unit and stirring unit constantly such that when the development operation ends and the developing apparatus is halted, the two-component developer is present in both the developing unit and stirring unit. When the developing apparatus is halted, the charge of the toner contained in the two-component developer gradually decreases. Accordingly, when the developing apparatus is left in a halted state for a long time, the toner contained in the developer turns into uncharged toner having substantially no charge. Similarly to charge-deficient toner, uncharged toner separates from the magnetic carrier easily, and therefore, when the development operation is resumed with uncharged toner present in the developing unit, toner scattering occurs in a similar manner to the charge-deficient toner described above, contaminating the interior of the apparatus. Furthermore, when uncharged toner exists in the developing unit, the developer is carried on the developer carrier by rotating the developer carrier, even if the development operation is not resumed, and as a result, the uncharged toner separates from the developer and scatters. Even when the developer is not carried to the developer carrier, the uncharged toner in the developing apparatus flies up when the developer containing the uncharged toner is conveyed or the developing apparatus receives an external impact, and as a result, the uncharged toner leaks out through gaps in the developing apparatus and scatters. The description provided here deals with toner scattering occurring due to the presence of uncharged toner in the developing unit, but toner scattering may also occur when charge-deficient toner having an insufficient charge, rather than uncharged toner, is present in the developing unit.
Further, in the developing apparatus of Prior Art 1, the two-component developer circulates between the developing unit and stirring unit constantly, and when the stirring unit is replenished with a large amount of toner following an operation to form an image having a high image density or the like, the two-component developer continues to be conveyed from the stirring unit to the developing unit. When a large amount of replenishment toner is provided, the proportion of uncharged toner in the two-component developer increases, and therefore a greater amount of time is required to stir the toner to the desired charge than when the amount of replenishment toner is comparatively small following the formation of an image having a low image area ratio. When a large amount of replenishment toner is provided and the two-component developer continues to be conveyed from the stirring unit to the developing unit in a similar manner to a case in which a small amount of replenishment toner is provided, two-component developer containing charge-deficient toner that has not been stirred sufficiently is conveyed to the developing unit. When two-component developer containing charge-deficient toner is conveyed to the developing unit, toner scattering occurs, contaminating the interior of the apparatus.
Meanwhile, in a developing apparatus using a two-component developer, as described above, the amount of developer carried on the developing sleeve is restricted to a suitable amount for development by a gap, or doctor gap, between a developer amount restricting member, which is disposed opposite the surface of the developing sleeve, and the developing sleeve. A magnetic brush constituted by the developer on the developing sleeve, which has been restricted to an amount suitable for development, is rubbed against or brought close to a photosensitive body in an appropriate state for development, and an image is formed by a development voltage. An example of this type of developing apparatus is described in Japanese Unexamined Patent Application Publication H11-194617 (Prior Art 2).
In this type of conventional developing apparatus, the normal direction magnetic flux density on the surface of the developing sleeve in an area near a doctor gap opposing the developer amount restricting member on the upstream side of the doctor gap in a developing sleeve surface motion direction takes a high value. In the developing apparatus of Prior Art 2, the maximum value of the magnetic flux density in a normal direction relative to the surface of the developing sleeve in an area near the doctor gap on the upstream side of the doctor gap in the developing sleeve surface motion direction takes a high value of 60 [mT]. As a result, the following problems arise.
When the magnetic flux density in a normal direction relative to the surface of the developing sleeve is high, a magnetic brush having a strong inter-carrier connection is formed on the surface of the developing sleeve. Further, when the developer passes through the doctor gap, i.e. the gap between the developer amount restricting member and the developing sleeve, in the area near the developing doctor on the upstream side thereof in the developing sleeve surface motion direction, the magnetic brush in a higher position than the doctor gap collides with the developer amount restricting member. When the magnetic brush having a strong inter-carrier connection collides with the developer amount restricting member, the carriers become less likely to separate, and therefore the developer particles and the developer amount restricting member, as well as the particles contained in the developer, rub together with a high degree of stress. When the developer is rubbed with a high degree of stress, the developer deteriorates, reducing the life of the developer. Hence, in consideration of the life of the developer, the stress received by the developer at the part where the developer amount restricting member and developing sleeve oppose each other must be reduced.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent No. 3,492,156, Japanese Unexamined Patent Application H04-198966, Japanese Unexamined Patent Application H08-278695, Japanese Unexamined Patent Application H11-184249, and Japanese Unexamined Patent Application 2002-244440.